calmlikeabomb :
675M =~ 670MX > 670M (+~20%) > 660M (+~15%-20%) > 650M (~+15%)
but the 660M and 650M are highly overclockable.
The 660m and 650m are overclockable... but so are all of them if you want to go down that road.
(Overclocking a laptop requires a lot of reading, tweaking time and attention to heat monitoring if you want to mitigate the dangers it can bring. For the most part it is entirely unrecommended for neophytes.)
Most of the time the 660m and 650m are built into laptops that are less optimal to overclocking.
Note, the more mobile a laptop becomes, the less room it has for cooling.
Furthurmore, the average 660m and 650m will never reach stock 670mx speeds.
The 670mx is actually slightly faster than the 675m in DX11 titles, and thus is roughly 50% faster than a 660m assuming everyone is at stock.
Furthurmore, the 670mx will be built into a laptop MUCH more suited to overclocking.
I know for certain a 675m (the 670mx is actually slightly faster) games at 1080p without much trouble. (There's an MSI GT70 with a 675m on a desk feet from me right now.) Sometimes you have to turn down to "high" instead of "ultra" for higher-end games. Every so often you run into games which require a turn-down to Medium.
The 650m does fairly well at 720p, but also has to turn down settings for certain titles. I easily prefer high at 1080p to ultra at 720p.
Since you seem to have it narrowed down...
The Sager 11" is fairly amazing for its size... it just that an 11" laptop is making sacrifices most people really don't need.
The 11" screen is fairly small... and is not particularly high quality even for most 720p screens. You lose the optical drive, and plugging multiple things in is a bit of a pain due to just how portable the laptop is. That being said, the little thing fits almost anywhere. A former co-worker told me his gets particularly hot while gaming. One really nice aspect is that most places will build it with the options you want.
The Lenovo Y480 looks like a fair balance of speed, price, and portability. At 4.85 lbs it is as light as anyone would really need it. It has reasonable stats, and the only real downside is the 5400rpm drive and the fact that Lenovo offers no options to swap it out. The screen gets mediocre reviews even among 720p screens. That being said, the Y480 comes out at a lower price point and an SSD addition and maybe external USB laptop HDD adapter would probaby easily fit in your budget, and you don't seem worried about the screen as much. I haven't seen on in person, but most of the reviews on the Lenovo units say they are well made overall.
You might also consider the Lenovo Y400... it has a 655m (I'd guess the 650m with SLI capabilities) and the laptop comes with a "ultrabay" which supposedly allows SLI 655s. (that option may or not be available yet as there are no listings of SLI Y400s.) It seems simply a Windows 8 refresh (over the Y480) with slightly newer hardware, but might be worth investigating.
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